make audio driver selection exclusive
fix #20677: playing position is not displayed in fret diagram
fix #20678: Stems go crazy when converting note head to Note.HEAD_SLASH
Instrument dialogue: when a new staff is added and/or is linked to another staff

1) It is not completely initialized (not from dlg item settings anyway)

2) it is not inserted in the linked staff list.
Merge pull request #288 from mgavioli/INSTR_DLG_linked_staves_and_proper_staff_init

INSTR. DLG: new or newly linked staved not managed correctly